R – Extract email addresses from a block of text

This explanation is based on a commented Ruby script from a friend of mine. If you want to improve the script, feel free to update it at the link.

First, note that when Ruby calls out to a shell, it typically calls /bin/sh, not Bash. Some Bash syntax is not supported by /bin/sh on all systems.

Here are ways to execute a shell script:

cmd = "echo 'hi'" # Sample string that can be used

1. Kernel#` , commonly called backticks – `cmd`

   This is like many other languages, including Bash, PHP, and Perl.

   Returns the result (i.e. standard output) of the shell command.

   Docs: http://ruby-doc.org/core/Kernel.html#method-i-60

   value = `echo 'hi'`
   value = `#{cmd}`
   

2. Built-in syntax, %x( cmd )

   Following the x character is a delimiter, which can be any character. If the delimiter is one of the characters (, [, {, or <, the literal consists of the characters up to the matching closing delimiter, taking account of nested delimiter pairs. For all other delimiters, the literal comprises the characters up to the next occurrence of the delimiter character. String interpolation #{ ... } is allowed.

   Returns the result (i.e. standard output) of the shell command, just like the backticks.

   Docs: https://docs.ruby-lang.org/en/master/syntax/literals_rdoc.html#label-Percent+Strings

   value = %x( echo 'hi' )
   value = %x[ #{cmd} ]
   

3. Kernel#system

   Executes the given command in a subshell.

   Returns true if the command was found and run successfully, false otherwise.

   Docs: http://ruby-doc.org/core/Kernel.html#method-i-system

   wasGood = system( "echo 'hi'" )
   wasGood = system( cmd )
   

4. Kernel#exec

   Replaces the current process by running the given external command.

   Returns none, the current process is replaced and never continues.

   Docs: http://ruby-doc.org/core/Kernel.html#method-i-exec

   exec( "echo 'hi'" )
   exec( cmd ) # Note: this will never be reached because of the line above
   

Here's some extra advice: $?, which is the same as $CHILD_STATUS, accesses the status of the last system executed command if you use the backticks, system() or %x{}. You can then access the exitstatus and pid properties:

$?.exitstatus

For more reading see:

• http://www.elctech.com/blog/i-m-in-ur-commandline-executin-ma-commands
• http://blog.jayfields.com/2006/06/ruby-kernel-system-exec-and-x.html
• http://tech.natemurray.com/2007/03/ruby-shell-commands.html

Using regular expressions is probably the best way. You can see a bunch of tests here (taken from chromium)

function validateEmail(email) {
    const re = /^(([^<>()[\]\\.,;:\s@"]+(\.[^<>()[\]\\.,;:\s@"]+)*)|(".+"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/;
    return re.test(String(email).toLowerCase());
}

Here's the example of regular expresion that accepts unicode:

const re = /^(([^<>()[\]\.,;:\s@\"]+(\.[^<>()[\]\.,;:\s@\"]+)*)|(\".+\"))@(([^<>()[\]\.,;:\s@\"]+\.)+[^<>()[\]\.,;:\s@\"]{2,})$/i;

But keep in mind that one should not rely only upon JavaScript validation. JavaScript can easily be disabled. This should be validated on the server side as well.

Here's an example of the above in action:

function validateEmail(email) {
  const re = /^(([^<>()[\]\\.,;:\s@\"]+(\.[^<>()[\]\\.,;:\s@\"]+)*)|(\".+\"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/;
  return re.test(email);
}

function validate() {
  const $result = $("#result");
  const email = $("#email").val();
  $result.text("");

  if (validateEmail(email)) {
    $result.text(email + " is valid :)");
    $result.css("color", "green");
  } else {
    $result.text(email + " is not valid :(");
    $result.css("color", "red");
  }
  return false;
}

$("#email").on("input", validate);

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

<label for=email>Enter an email address:</label>
<input id="email">
<h2 id="result"></h2>